sqlalchemy flask: AttributeError: 'Session' object has no attribute '_model_changes' on session.commit()

Question

I've seen a lot of problems with SessionMaker, but this one is slightly different. Not sure why, but sqlalchemy won't let my session object commit.

In my app, I have some code that does:

views.py

rec = session.query(Records).filter(Records.id==r).first()
n = rec.checkoutRecord(current_user.id)
session.add(n)
session.commit()

models.py:

class Records(UserMixin, CRUDMixin, Base):
    __table__ = Table('main_records', Base.metadata, autoload=True)


    def checkoutRecord(self,uid):
        self.editing_uid = uid 
        self.date_out = datetime.now()
        return self

    def checkinRecord(self,uid):
        self.editing_uid = uid 
        self.date_in = datetime.now()
        return self

The program craps out on the commit(), giving the above exception. Interestingly, some test code which does not import flask, but does import sqlalchemy works fine and lets me commit without error.

The full stack-trace:

Traceback (most recent call last):
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/flask_login.py", line 663, in decorated_view
    return func(*args, **kwargs)
  File "/Users/bhoward/projects/PeerCoUI/mk2/peercoui/app/records/views.py", line 65, in select_view
    session.commit()
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 149, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 721, in commit
    self.transaction.commit()
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 354, in commit
    self._prepare_impl()
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 323, in _prepare_impl
    self.session.dispatch.before_commit(self.session)
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/event.py", line 372, in __call__
    fn(*args, **kw)
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/flask_sqlalchemy/__init__.py", line 162, in session_signal_before_commit
    d = session._model_changes
AttributeError: 'Session' object has no attribute '_model_changes'

Full code for the project is in github: https://github.com/bhoward00/peercoui

Any advice appreciated

Answer 1

Yes this is exactly problem when using flask-sqlalchemy models mixed with pure sqlalchemy session. Thing is that flask-sqlalchemy subclasses the base Session from sqlalchemy and adds some internals one of which is the _model_changes dict. This dict is used for model modification tracking.

So if you want to use flask-sqlalchemy based models with regular sqlalchemy session, one way would be to just add the dict to the session (this is just example code):

def create_session(config):
    engine = create_engine(config['DATABASE_URI'])
    Session = sessionmaker(bind=engine)
    session = Session()
    session._model_changes = {}
    return session 

I had the same exact problem as you, so hopefully this should help you.

UPDATE:

There is new version available, which should be fixing this behaviour, quoting the 2.0 docs:

Changed how the builtin signals are subscribed to skip non Flask-SQLAlchemy sessions. This will also fix the attribute error about model changes not existing.

Docs: http://flask-sqlalchemy.pocoo.org/2.0/changelog/#version-2-0

Answer 2

I had the same problem as well and solved it by modifying the _SessionSignalEvents class within __init__.py in flask-sqlalchemy. However, I just noticed that such a fix had already been in place since 8 months on the official repository.

If you encounter a similar problem, I would recommend you to pull the latest version of the project from github (https://github.com/mitsuhiko/flask-sqlalchemy/) since the one currently available through pip install is outdated.